Voltages in a circuit may be measured against a reference voltage that is a known quantity. A bandgap circuit may produce such a voltage reference. FIG. 1 is a block diagram of a bandgap circuit 102 coupled with a start-up circuit 100. Temperature-independent biasing and low temperature coefficient reference voltages are often required in integrated circuit design. Band-gap reference is a circuit which generates a low temperature coefficient reference by summing base emitter voltage (V.sub.BE) of a bipolar transistor and a weighted V.sub.T, where V=voltage and T=temperature. Once the bandgap voltage reference has been established, all other voltages may be measured against the reference voltage.
The bandgap circuit 102 is typically coupled with a start-up circuit 100. Typically, the main purpose of the start-up circuit 100 is to start the bandgap circuit 102. The start-up circuit 100 may ensure that the bandgap circuit 102 operates within a valid operating point. As source voltage (V.sub.dd) ramps from zero volts to a final value, such as 5V, the bandgap circuit 102 should reach its final value as well. Since it is possible for the bandgap circuit 102 to remain at zero current and zero voltage, one of the start-up circuit's functions is to ensure that the bandgap circuit 102 does not remain at zero current and zero voltage.
Such a combination of bandgap circuit 102 and start-up circuit 100 may be used for various applications. For example, these types of circuits may be used in a digital to analog converter or an analog to digital converter.
A potential problem for the start-up circuit 100 is that it tends to draw excessive current. The conventional start-up circuit 100 typically requires a current of approximately 100 micro Amps during and after the bandgap circuit reaches its typical goal value of approximately 1.25V. It would be desirable to reduce the current requirements of the start-up circuit 100 since, typically, low power circuits are more reliable than high power circuits. Additionally, if the combination of the start-up circuit 100 and the bandgap circuit 102 were used in an application requiring a battery, the limited power available through the battery may quickly expire when used with a circuit having high current requirements. Another potential problem is the heating of the circuit due to the high current required by the conventional start-up circuit 100. Since many integrated circuits include devices that are in close proximity to each other, it is typically desirable to run circuits at a relatively low current for heat management purposes.
It would be desirable for a start-up circuit to run on less current and, accordingly, be more efficient, more reliable, and have better heat management. The present invention addresses such a need.